1. Field of the Invention
The field of the invention relates generally to transmissions, and more particularly the invention relates to continuously variable planetary gear sets that can be used in transmissions as well as other industrial and land, air and water-borne vehicles.
2. Description of the Related Art
In order to provide a continuously variable transmission, various traction roller transmissions, in which power is transmitted through traction rollers supported in a housing between torque input and output discs, have been developed. In such transmissions, the traction rollers are mounted on support structures which, when pivoted, cause the engagement of traction rollers with the torque discs in circles of varying diameters depending on the desired transmission ratio.
However, the success of these traditional solutions has been limited. For example, in one solution, a driving hub for a vehicle with a variable adjustable transmission ratio is disclosed. This method teaches the use of two iris plates, one on each side of the traction rollers, to tilt the axis of rotation of each of the rollers. However, the use of iris plates can be very complicated due to the large number of parts that are required to adjust the angular position of the iris plates during shifting of the transmission. Another difficulty with this transmission is that it has a guide ring that is configured to be predominantly stationary in relation to each of the rollers. Since the guide ring is stationary, shifting the axis of rotation of each of the traction rollers is difficult.
A key limitation of this design and improvements of this design is the absence of means for generating and adequately controlling the axial force acting as normal contact force to keep the input disc and output disc in sufficient frictional contact against the balls as the speed ratio of the transmission changes. Due to the fact that rolling traction continuously variable transmissions require various magnitudes of axial force at various torque levels and speeds in order to prevent the driving and driven rotating members from slipping on the speed changing friction balls, where a constant level of axial force is applied, excessive force is applied when torque transmission levels are lower. This excessive axial force lowers efficiency and causes the transmission to fail significantly faster than if the proper amount of force was applied for any particular gear ratio. The excessive force also makes it more difficult to shift the transmission. Improvements in the field of axial force production have been made but further advances are required.
Further improvements have been developed for the increased performance and efficiency of continuously variable transmissions. There is a need to incorporate these improvements into an advanced design for a continuously variable transmission.